This invention relates to fuel cells, and more particularly to methods and apparatus for the improved delivery of input reactants to fuel cells.
Many types of fuel cells are known in the art, such as solid oxide, molten carbonate, phosphoric acid and proton exchange membrane (PEM) fuel cells. Fuel cells generate electricity by directly converting chemical energy to electrical energy. In a typical fuel cell, an electrolytic medium separates an anode and a cathode. A voltage is produced between the anode and cathode when a fuel is introduced to the anode, an oxidant is introduced to the cathode and the cell is maintained within the correct temperature range. The electrolytic medium allows an ionic species to travel between the cathode and the anode.
The reaction products generated by the fuel cell are relatively simple and benign, typically including water and carbon dioxide, thus minimizing environmental concerns. In contrast with fossil fuel based power sources, such as the internal combustion engine, fuel cells are simpler, quieter, nonpolluting and have high operating efficiencies. For these and other reasons, fuel cells are considered promising power sources for the future.
In practice, however, a fuel cell power plant can be complex. Considerable hardware may be required to support the fuel cells, which are typically arranged in an electrically-connected cell stack assembly (CSA). Such hardware can include a thermal management subsystem for maintaining the cell stack assembly at the proper temperature, a water management subsystem for handling water generated as a reaction product of operating the cell stack assembly and for maintaining proper humidity throughout the power plant, a fuel subsystem for processing and delivering the fuel reactant to the cell stack assembly, and a blower for delivering the oxidant to the cell stack assembly.
FIG. 1 illustrates a typical fuel cell power plant 1 including a fuel cell stack assembly 3. Reference numeral 5 generally indicates the components of a typical fuel processing subsystem, and reference numeral 7 indicates components of at typical thermal management subsystem. The thermal management subsystem 7 can also include provision for water management, such as provision for the recycling of water that is generated as a reaction product of operating the fuel cell stack assembly 3. The inverter 8, if required, converts the d.c. output of the fuel cell stack assembly 3 to a.c. for provision to an electrical power grid. For some applications, such as powering an electric motor in an automobile, a motor drive replaces the inverter. The fuel cell power plant 1 is merely exemplary, and, as understood by one of ordinary skill in the art, the components and subsystems of a fuel cell power plant 1 can vary depending on the application - a phosphoric acid stationary power plant for industrial use will differ from a mobile - (PEM) power plant. Furthermore, a mobile PEM power plant that can be provided with hydrogen as a fuel reactant can differ considerately from a PEM plant for installation in an automobile, which can be required to include a subsystem for producing hydrogen fuel from gasoline. In general, a fuel cell power plant includes those subsystem components necessary for the application for which the power plant is to be used, and that are appropriate to the type of fuel cells incorporated by the fuel cell power plant.
The delivery of input reactants is particularly important in a fuel cell stack assembly. The rate of the delivery of the input reactants affects the power output and efficiency of each of the individual cells that make up the cell stack assembly and also the amount of thermal energy that must be removed from a particular portion of the cell stack assembly to maintain the proper operating temperature. Typically, the oxidant input reactant is delivered by a single blower, such as the single blower 9 in FIG. 1, which provides oxidant at slightly above ambient pressure to all the individual cells of the fuel cell stack assembly 3. The input oxidizer reactant is delivered at a rate that, on average, provides the proper operating stochiometry, temperature and power output of the cell stacks that make up a typical fuel cell stack assembly 3.
However, use of a single blower is not entirely satisfactory. The blower is usually fairly large, consumes considerable power, and can be noisy in operation. Furthermore, all the cells of the cell stack assembly are serviced by the same blower, and hence the performance thereof is more difficult to individually enhance.
Accordingly, it is an object of the invention to provide improved methods and apparatus for the delivery of input reactants to fuel cells.
According to one aspect, the invention provides a fuel cell stack assembly for use in a fuel cell power plant and for producing electricity from fuel and oxidant reactants. The fuel cell stack assembly includes a plurality of individual fuel cells each having an electrolyte, cathode and anode, and the cell stack assembly is adapted for defining anode flow fields for exposing the anodes to a fuel, cathode flow fields for exposing the cathodes to an oxidant, and for preventing the commingling of the fuel and oxidant reactants between adjacent anodes and cathodes. Also included are input and output manifolds in fluid communication with the cathode flow fields, and at least one blower mounted with one of the manifolds for flowing oxidant through the cathode flow fields.
The blower can be mounted with one of the manifolds, such as within the inner volume defined by the manifold, typically at or near a manifold wall, and can be a vane axial or centrifugal blower, and can be driven by a variable speed motor. Preferably, the blower is positioned in or near a manifold wall and facing the cells of the cell stack assembly. Multiple, blowers can be mounted with the manifolds of a cell stack assembly, and can either push or pull, or both, the oxidant through the cathode flow fields.
According to another aspect, the invention provides an improved fuel cell power plant having a plurality of fuel cell stack assemblies. Each cell stack assembly includes a plurality of individual fuel cells and has intake and exhaust manifolds in fluid communication with the cathode flow fields of the assembly for providing and removing oxidant from the stack assemblies. The improvement includes a plurality of blowers, each of which is associated with one of the plurality of fuel cell stack assemblies for flowing oxidant in the cathode flow fields thereof. A plurality of sensors can be included for sensing operating characteristics of the fuel cell stack assemblies, and the invention can include a controller in electrical communication with the sensors and the blowers for controlling the delivery of oxidizer by the blowers responsive to the sensors. Sensors can be of several types, and can include sensors for sensing temperature, voltage, current, oxygen concentration and humidity. A particularly useful sensor to employ is an oxygen concentration sensor located for sensing an oxygen concentration in or related to the cathode flow fields, as the output of the fuel cells is directly proportional to this oxygen concentration.
The invention also includes methods for delivering input reactants to a fuel cell stack assembly including a plurality of individual fuel cells and adapted for providing anode and cathode flow fields for exposing the anodes and cathodes of the individual fuel cells to a reducing and oxidant reactants, respectively. The method includes the steps of providing oxidant input and exhaust manifolds in fluid communication with the cathode flow fields; mounting at least one blower with one of the manifolds for controlling the flow of the oxidant through the cathode flow fields; and operating the blower to selectively flow the oxidant to the cathode flows fields for controlling one of the temperature, voltage, current, oxygen concentration and the electrical power output of the fuel cell stack assembly of selected fuel cells thereof. For example, the method can also include determining a temperature characteristic of the fuel cell stack assembly and controlling the blower responsive to the temperature. The step of controlling can include increasing the flow of the oxidizer when the temperature is below approximately a selected temperature and reducing the flow rate when the temperature is above approximately at selected temperature.
In yet another aspect, a method practiced in accordance with the invention for providing oxidant input reactants to a fuel cell power plant having a plurality of fuel cell stack assemblies includes the steps of providing a plurality of blowers; associating the blowers with the fuel cell stack assemblies for flowing oxidant in the cathode flow fields of the assemblies such that each blower is associated with a fuel cell stack assembly; and operating the blowers for flowing oxidant through the flow fields of the fuel cell stacks.
These and other features of the invention are more fully set forth with reference to the following detailed description, and the accompanying drawings.